I. Field of the Invention
The present invention relates to an improved firing circuit capable of reducing the effects of non-synchronous, high frequency line perturbations on its output, and more particularly, to such a firing circuit for use as an intensity selector for a gaseous discharge lamp dimming system.
II. Description of the Prior Art
Dimming systems for gaseous discharge lamps, and in particular, fluorescent lamps, generally utilize an auxiliary circuit in the form of a power switch which is controlled by an intensity selector. The power switch, usually a thyristor, supplies current to the lamp or lamps and secures various illumination or dimming levels by controlling the interval of current conduction through the lamps during each half cycle of the power supply. The intensity selector which controls the thryistor is made selectively variable to provide continuous adjustment of level of illumination. One such dimming system is disclosed in U.S. Pat. No. 3,863,102 -- Herzog, assigned to the General Electric Company and another is disclosed in U.S. Pat. No. 3,767,940 -- Herzog et al, also assigned to the General Electric Company.
A major problem occurring in such dimming systems for fluorescent lamps has been, and is, flicker. Flicker is a phenonenon which results from an instability or variability, regularly or irregularly, of light level and may be quite annoying to the viewer. Flicker may occur at both high and low light levels as well as at any intermediate light level.
This flicker problem is addressed in the aforementioned U.S. Pat. No. 3,863,102 patent wherein at least one source of such flicker was recognized and solved, that being in the auxiliary circuit. Another source of flicker was addressed and essentially obviated in U.S. Pat. No. 3,935,502 -- Herzog, also assigned to the General Electic Company. Therein flicker was found to be caused by the ballast wherein non-symmetrical half cycles of lamp current occurred, this because one end of the lamp was grounded through a heater winding while the other side was allowed essentially to float.
For years, however, flicker has persisted in certain installations. Through much experimentation and study, it was found that where a fluorescent lamp dimming system is used at a location where a supply line of high impedence feeds both the dimming system and a relatively large induction motor such as found in a heat pump, air conditioner or compressor, and both lines are concurrently in use, a non-synchronous, high frequency, substantially sinusoidal signal is imposed upon the AC line. This high frequency ripple produces varying AC power line zero-crossing times which result, in turn, in unequal "on" times for the lamps; the eye interprets this as flicker. If this zero-crossing time varies 50 microseconds (out of 8330 microseconds for 60 Hz), the results are objectionable.
It is desirable, therefore, to provide a dimming system for gaseous discharge lamps, and in particular, fluorescent lamps, wherein the visual effects of non-synchronous, high-frequency AC line perturbations are minimized.
Accordingly, it is an object of the present invention to provide a firing circuit capable of use as an intensity controller for such a dimming system wherein means are provided for eliminating the visual effects of such AC line perturbations.